Molecular dynamics (MD) simulations and experiments were carried out to study the glass transition temperature of glycidyl azide polymer (GAP) which is widely used in propellants as an energetic binder. A modification method was applied to improve the mechanical properties of GAP at low temperatures, through the addition of energetic ionic liquid plasticizers to the polymer matrix. The design and simulate under the COMPASS force field, with the ensembles of constant particle number, pressure, temperature (NPT) done. The variation of density and free volume of polymers was observed with decreasing the temperature, and the glass transition temperatures (Tg) were estimated based on the variation of density and free volume vs. temperature. Simulation and experimental results revealed that the addition of energetic ionic liquid plasticizer to the polymer matrix reduced Tg. Also, by changing the anion of ionic liquid, the size of the molecule and the number of atoms, the value of Tg was changed. The variations were 235 K (GAP), 233 K (GAP/IL-N-3), 230 K (GAP/IL-NO3) and 214 K (GAP/IL-DCA).